Communication devices such as wireless devices are also known as e.g. User Equipments (UE), mobile terminals, wireless terminals and/or mobile stations. Wireless devices are enabled to communicate wirelessly in a wireless communications system or wireless communication system, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two wireless devices, between a wireless device and a regular telephone and/or between a wireless device and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the wireless communications system.
Wireless devices may further be referred to as mobile telephones, cellular telephones, or laptops with wireless capability, just to mention some further examples. The wireless devices in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as wireless device or a server.
The wireless communications system covers a geographical area which is divided into cell areas, wherein each cell area being served by a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the area of radio coverage provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the wireless devices within range of the base stations.
In some RANs, several base stations may be connected, e.g. by landlines or microwave, to a radio network controller, e.g. a Radio Network Controller (RNC) in Universal Mobile Telecommunications System (UMTS), and/or to each other. The radio network controller, also sometimes termed a Base Station Controller (BSC) e.g. in GSM, may supervise and coordinate various activities of the plural base stations connected thereto. GSM is an abbreviation for Global System for Mobile Communications (originally: Groupe Spécial Mobile).
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
UMTS is a third generation mobile communication system, which evolved from the GSM, and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for wireless devices. The 3GPP has undertaken to evolve further the UTRAN and GSM based radio access network technologies.
According to 3GPP GSM EDGE Radio Access Network (GERAN), a wireless device has a multi-slot class, which determines the maximum transfer rate in the uplink and downlink direction. EDGE is an abbreviation for Enhanced Data rates for GSM Evolution. In the end of 3008 the first release, Release 8, of the 3GPP Long Term Evolution (LTE) standard was finalized and later releases have also been finalized.
In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the mobile station to the base station.
In cellular telecommunications, the term handover refers to the process of transferring an ongoing call or data session from one cell to another and it may imply for some deployment scenarios transferring from one network node, i.e., a primary cell served by a primary node or source network cell served by a source node, connected to a core network, to another cell or network node, i.e., a neighbour cell or target cell served by a target network node. In a typical wireless communications network, one network node may serve many cells and each cell typically only covers a limited geographical area with some overlap; therefore, handover becomes a very important feature for the seamless mobility of wireless devices in the entire wireless communications network. The performance of handover also becomes an important factor that affects the user's experience and the amount of radio resources used for the connection. One main purpose of handover is to make sure the UE connection is always maintained by being served by the best frequency and cell.
In the Long Term Evolution (LTE) Radio Access Technology (RAT), a wireless device, e.g., a user equipment (UE) in Radio Resource Control_Connected (RRC_CONNECTED) mode, measures the signal strength, e.g. the Reference Symbol Received Power (RSRP), or the signal quality, e.g., Reference Symbol Received Quality (RSRQ), of the serving cell and neighbour cells, as schematically depicted in an example in FIG. 1. The wireless device reports the measurement results in a measurement report, either periodically or when they fulfil event criteria as defined by a network node, e.g., an eNB, such as the primary network node, according to, for example, 3rd Generation Partnership Project (3GPP) standard 36.331. One event criterion, event A5, is fulfilled when the neighbour cell, measured on a specific frequency and a Physical Cell Identity (PCI) on that frequency, gets stronger than an absolute threshold (Threshold 2) at the same time as the primary serving cell gets worse than another absolute threshold (Threshold 1) for at least a minimum amount of time, i.e. Time To Trigger. Optionally the primary network node may configure the wireless device to use Cell Individual Offset (CIO) added for a certain PCI, i.e., Threshold+Hysteresis+CIO. One event criterion, event A3, is fulfilled when the neighbour cell, measured on a specific frequency and a Physical Cell Identity (PCI) on that frequency, gets a certain amount stronger than the primary or serving, cell, i.e., the Offset+Hysteresis, for at least a certain minimum time, i.e., the Time to trigger. Optionally the primary network node may configure the wireless device to use Cell Individual Offset (CIO) added for a certain PCI, i.e., Offset+Hysteresis+CIO. This event is typically used as trigger for handover. That is, a handover is initiated by the primary network node after receiving a measurement report from the wireless device that indicates that at least one neighbour cell, e.g., PCI, fulfils the event criterion. There might be cases when several neighbour cells fulfil this criterion. If this happens, the wireless device sends a list, including these cells arranged in signal strength order, in the measurement report. The measurement report also contains information about measured RSRP and RSRQ for the primary cell cell and optionally RSRP and RSRQ measured values for the reported neighbour cells, e.g., PCIs. The primary network node selects the appropriate cell as neighbour (i.e., target) cell for handover, and then translates the wireless device-reported PCI to a NW address and NW network node and cell identity for handover preparation signalling purposes.
However, it is a problem in conventional networks that handover is sometimes triggered when not really necessary, resulting in wasted network resources, or that handover may be triggered too late, resulting in a dropped connection.